President Bush has launched plans for a new series of U.S. space missions that may eventually include human flights to Mars.

President Bush says the U.S. space agency has a new focus and clear objectives to continue exploring the solar system in a new generation of spacecraft.

"We chose to explore space because doing so improves our lives and lifts our national spirit, so let us continue the journey," he said.

First on the president's new space agenda is completing work on the International Space Station by 2010. Mr. Bush says he wants to fulfill the U.S. commitment to 15 partner countries and re-focus onboard research toward overcoming the effects of human space flight.

"The environment of space is hostile to human beings," he said. "Radiation and weightlessness pose dangers to human health and we have much to learn about their long-term effects before human crews can venture through the vast voids of space for months at a time."

Finishing the space station means NASA will relaunch some of its existing space shuttles which have been grounded since last year's loss of the shuttle Columbia and the death of its seven astronauts.

But by the end of the decade, the president says the entire space shuttle fleet will be retired after nearly 30 years of service. In its place, will be a new generation of spacecraft, called the Crew Exploration Vehicle, which Mr. Bush says should be ready for launch by 2014.

The president says astronauts will use those vehicles to travel to the space station and return to the moon as early as 2015. After a series of robotic surveys on the moon, Mr. Bush adds astronauts will eventually establish some sort of base there for scientific research and as a launching pad for future exploration, including a trip to Mars.

"Lifting heavy spacecraft and fuel out of the earth's gravity is expensive," he said. "Spacecraft assembled and provisioned on the moon could escape its far-lower gravity using far less energy and thus far less cost."

Unlike the space race of the Cold War, President Bush says space is now more of a journey that the United States intends to pursue with other nations in a spirit of cooperation and friendship.

That should help offset some of the considerable expense of the program, which over the next five years alone will cost $12 billion. The White House says the bulk of that funding will come from reallocating $11 billion of NASA's current $86 billion budget.

The president will ask Congress for an additional $1 billion for NASA's existing five-year plan. Some Congressional critics charge the new mission is too expensive at a time when the country is still paying to rebuild Iraq and fight terrorism.

White House officials counter that NASA spends less than one percent of the federal budget and say new technologies developed for space exploration have often had commercial applications including kidney dialysis, programmable heart pacemakers, and surgical probes.

Other critics say the president's plan is tied to election year politics and the need to present a vision more inspiring than fighting terrorism. Much of the immediate financial windfall of developing new spacecraft will come in the politically-important states of Florida and Texas.

White House spokesman Scott McClellan dismissed suggestions of playing politics with space, saying the decisions came from a comprehensive review of the direction of the U.S. space agency following the loss of the shuttle Columbia and reflects the president's commitment to continuing exploring space.